The Hubble constant H is one of the most important numbers in
cosmology
because it may be used to estimate the size and
age of the Universe.
It indicates the rate at which the universe
is expanding. Although the Hubble "constant" is not really constant because it
changes with time (and therefore should probably more properly be called the "Hubble
parameter"). The Hubble constant is often
written with a subscript "0" to denote explicitly
that it is the value at the present time, but
we shall not do so.

FIGURE: The image to the right,
taken with the
Wide-Field and
Planetary Camera (WFPC2) of the
Hubble Space Telescope
shows many galaxies
(most of the fuzzy patches are galaxies containing billions of stars) that are
billions of light years away and are receding from us at high velocities.
Click on the image to get a larger version (WARNING: 300 kB).

The Hubble Expansion Law

In 1929, Edwin Hubble announced that
almost all galaxies appeared to be moving away from us. This
phenomenon was observed as a
redshift of a galaxy's spectrum.
This redshift appeared to have a
larger displacement for faint, presumably further, galaxies. Hence, the
farther a galaxy, the faster it is receding from Earth.
The Hubble constant is given by

H = v/d

where v is the galaxy's radial outward velocity,
d is the galaxy's distance from earth, and H
is the current value of the Hubble constant.

Determining the Hubble Constant

Obtaining a true value for H is complicated. Two measurements are required.
First, spectroscopic observations reveal the
galaxy's redshift, indicating its radial velocity. The second measurement,
the most difficult value to determine, is the galaxy's precise distance from
Earth.
The value of H itself must be
derived from a sample of galaxies that are far enough away that motions
due to local gravitational influences are negligibly small (these are called
peculiar motion, and they represent deviations from the Hubble Law).

Units for Hubble's Constant

The units of the Hubble constant are "kilometers per second per
megaparsec." In other words, for each
megaparsec of distance, the velocity
of a distant object appears to increase by some value.
For example, if the Hubble constant was
determined to be 50 km/s/Mpc, a galaxy at 10 Mpc would have a redshift
corresponding to a radial velocity of 500 km/s.

Current Value of the Hubble Constant

The value of the Hubble constant initially obtained by Hubble was
around 500 km/s/Mpc, and has since been radically revised because initial
assumptions about stars yielded underestimated distances.
For the past three decades, there have been two major lines of investigation
into the Hubble constant. One team, associated with Allan Sandage of the
Carnegie Institutions, has derived a value for H around 50 km/s/Mpc. The
other team, associated with Gerard DeVaucouleurs of the University of
Texas, has obtained values that indicate H to be around 100 km/s/Mpc.